Quartz crystal sensors have been widely used to monitor vacuum deposition processes and to accurately control the amount of material that is deposited as well as the rate of deposit onto a surface, such as a substrate used for semiconductor, optics or display processing. In these sensors, one more piezoelectric crystals which can be manufactured from quartz, barium titanate, or another suitable material, are connected into a resonance circuit so that that the natural resonant frequency of one crystal can be monitored, the crystal being positioned in relation to the interior of the processing chamber. The natural resonance is primarily dependent upon total mass and geometry of the crystal, wherein the resonance frequency drops in relation to the amount of material that is coated onto the crystal during a vacuum deposition process. However, as the deposited material builds up, the sharpness of the composite resonance diminishes, and eventually a point is reached in which the crystal cannot adequately monitor the process accurately or effectively. At this point, the piezoelectric crystal must be replaced.
A number of multi-crystal sensor heads have been developed, including those described by U.S. Pat. No. 5,025,664, for use in multi-crystal quartz oscillator deposition monitors and in which a plurality of crystals can be rotated about a rotary carousel. Other versions have been developed, such as described, for example, in U.S. Pat. Nos. 4,362,125 and 3,383,238. Each of the foregoing designs, however, are typified by a two-dimensional structure that outwardly increases the overall size of the sensor head as the number of crystals increases.
According to one aspect, there is provided a high capacity piezoelectric crystal deposition sensor for use in detecting and monitoring changes in a deposited material in a processing chamber, said sensor comprising:
a three dimensional storage structure having a primary axis extending between respective first and second ends and an exterior lateral surface between said first and second ends extending about said primary axis;
a plurality of piezoelectric crystals supported by said storage structure at spaced positions along said exterior lateral surface; and
a drive mechanism for rotating and axially advancing or retreating said storage structure relative to the primary axis such that at least one piezoelectric crystal can be advanced relative to at least measuring position and a mechanism for electrically exciting the at least one advanced crystal and enable detection due to accumulating material on said crystal.
According to another aspect, there is provided a three-dimensional storage structure for use in a crystal deposition sensor, said three-dimensional storage structure comprising:
a carrier body having a primary axis extending between a first end and a second opposite end, the carrier body further having a lateral exterior surface defined between the first and second ends and extending about the primary axis; and
a plurality of retaining cavities disposed at spaced axial positions along the lateral exterior surface of the carrier body, each retaining cavity being sized for retaining a piezoelectric monitor crystal.
According to yet another version, there is provided a method of exchanging crystals used in a crystal deposition monitor, said method comprising the steps of:
disposing a plurality of piezoelectric crystals in spaced relation onto the exterior surface of a three-dimensional storage structure;
axially and rotatably advancing or retreating the storage structure in order to index at least one piezoelectric crystal relative to at least one measuring position;
electrically exciting the advanced piezoelectric crystal;
detecting the change of mass of material applied to a substrate;
axially and rotatably advancing or retreating the storage structure along a primary axis of the three-dimensional storage structure in order to index at least one other monitor crystal into the at least one sensor aperture.
The herein described storage structure can be used for the process control of the growth rate and associate thickness determination for an applicable process of specific materials onto a substrate in which the material is transported by one of several means from a material source to the substrate. The transport may be through media that may be essentially a vacuum or any rarified or pressurized media, as required by the process.
Examples of the foregoing can include vacuum evaporation that involves material transport from a hot source of material through a vacuum to the substrate. There are also many various means of sputtering in which ionized atoms of a gas intervening between the source and the substrate are used to energetically impinge upon a target material and the resulting eroded material fragments pass through the gas mixture to the substrate and substantially adhere to the substrate. Other less common means of material transport include laser induced ablation of the target, plasma ionization and arc deposition. All of these methods commonly include a step of transport of the material from a source (or target) through a vacuum or other process media and finally impingement and collection onto a substrate or onto a monitor crystal being used as a surrogate substrate.
A high capacity crystal exchange mechanism is therefore described that utilizes a moving three-dimensional monitor crystal storage element or storage structure. All stored monitor crystals have individual fixed locations and are arrayed on a complexly shaped surface, such as a cylinder or other polygon, in a predetermined arrangement. The particular array of monitor crystal locations chosen should be designed to facilitate the efficient movement of monitor crystals relative to at least one measuring position. In a preferred version, the three-dimensional storage structure is a cylindrical body having the monitor crystals retained in helical sequential form on a curved surface. Individual monitor crystals may be positioned at least one at a time into a position useful for receiving the flow of a deposition material, in which the crystals can be retained directly or within individual monitor crystal packages that facilitate the loading and unloading of the storage structure. In one version, motion of a point on the surface of the storage structure during sequential movement of monitor crystals into the working position is helical in nature and in which movement of the three-dimensional storage structure induces a change in the monitor crystal. This movement may be arbitrary in nature with sufficiently complex motion, inducing mechanisms that include one or more stepper motors or other suitable actuators, wherein the drive mechanism causes both rotational and axial movement about the primary axis of the storage structure.
One advantage provided by the herein described apparatus is that a high quantity monitor crystal storage capacity can be achieved in a low volume package, and in which greater numbers of crystals can be retained as opposed to conventionally known sensor heads utilizing two-dimensional crystal storage.
Another advantage is the ability to test cycle the storage structure in order to verify acceptable electrical operation of all piezoelectric crystals prior to closing the vacuum system and their subsequent use during deposition.
Yet another advantage is that the herein described apparatus enables a modular design approach that facilitates and eases the development of different capacity versions to meet the quantities of piezoelectric crystals required for a wide variety of processes that have adequate reserve capacity to meet desired maintenance intervals.
According to one version, a core mechanism can be provided for making electrical contact to the crystals and executing the organized flow of piezoelectric crystals into the measurement position. This core measurement and transport component will easily interface with various drive mechanisms for powering the crystal exchange process. Attachable drive mechanisms can include both in vacuum and non-vacuum drive power sources.
The system design can include a fluid based mechanism for extracting (or adding) heat to the overall exchanger mechanism in order to stabilize the retained crystal's temperature. The foregoing mechanism will extract the absorbed heat of radiation, the heat of condensation from the deposition sources, losses from drive motors or other ancillary sources of process heating.
The monitor crystals can also be retained for measuring other system parameters, such as temperature.
Advantageously, an apparatus including the present design is not dependent on any particular monitor crystal's size or shape. As such, crystals having a smaller area can be provided to the extent the crystal's vibration is confined to a central area and isolated from contact points (i.e., edges) and wherein the crystal face is not planar; for example, about a spherical surface.
Because smaller crystals can be used, a lower volume exchanger can be provided for the same number of monitor crystals.
In addition, another advantageous feature is the ability to easily replace monitor crystals within the defined structure.
These and other features and advantages will be readily apparent from the following Detailed Description, which should be read in conjunction with the accompanying drawings.